The purpose of cellular respiration is to generate energy in the form of certain molecules (ATP being the major one). There are actually several processes for creating energy in cells. Oxidative Phosphorylation, the Citric Acid Cycle, and Glycolysis are the three big ones.

Wikipedia has entries on all three of these and there are tons of web sites that cover them. Just do a google search on those words.

The energy is then used in various reactions within cells. The vast majority, I believe, goes to building proteins for various functions inside and out of the cell.

Great explanation! The questions 'where is it come from' and 'where is it transferred', with respect to energy could be answered in different contexts. The energy generated by cellular respiration comes precisely from the transformation of substrates (fatty acids if oxygen is involved or glucose if oxygen is not necessarily present). Where is it transferred? That depends on the particular process as well, but in the most general sense, every reaction that takes place in the body needs energy, even those that are involved in energy production (i.e. they required a small investment of ATP in order to produce more of it). The energetic currency, ATP is the essence of what's call 'reaction coupling', in which the energy (ATP) produced in one process is used to make another one happen.

Science is a wonderful thing if one does not have to earn one's living at it. (A. Einstein)